1. Technical Field of the Invention
The present invention relates generally to a gas concentration measuring apparatus which may be used in measuring the concentration of a preselected component of exhaust emissions of automotive engines, and more particularly to such a gas concentration measuring apparatus designed to ensure high resolution in measuring the concentration of gas over a desired range.
2. Background Art
As a typical one of the above type of gas concentration measuring apparatuses, an automotive air-fuel ratio measuring apparatus is known which works to measure the concentration of oxygen (O2) contained in exhaust emissions of motor vehicle engines as indicating an air-fuel ratio of a mixture. The result obtained is used in an air-fuel ratio control system consisting of an engine ECU etc. Stoichiometric burning controls to bring the air-fuel ratio near the stoichiometric air-fuel ratio under feedback control and lean-burn controls to bring the air-fuel ratio to within a given lean range under feedback control are being developed. In recent years, emission regulations or on-board diagnostic (OBD) requirements have been increasingly tightened. Improvement of the stoichiometric burning controls is, thus, being sought. Additionally, there is an increasing need for expanding an air-fuel ratio measurable range up to an atmospheric range as well as the lean range that is a typical air-fuel ratio controlling range. For instance, a sensor malfunction monitoring system is known to meet the OBD requirements which works to monitor a deterioration of a gas sensor such as clogging resulting in a decrease in sensor output current during a fuel cut-off (i.e., when exhaust gasses are equivalent to air) under a given operating engine condition. It is also essential to improve fuel efficiency as well as exhaust emissions. It is further essential to feedback-control a rich mixture at high load engine operating conditions.
Typically, lean-burn engines having a NOx occluding/reducing catalyst installed in an exhaust system encounter the problem in that a large amount of NOx is occluded in the catalyst during a lean-burn engine operation, which results in lowered ability of absorbing NOx emissions. Additionally, typical fuel contains sulfur, thus poisoning the NOx occluding/reducing catalyst. In order avoid these problems, the rich air-fuel ratio controls have been implemented to recovery the NOx absorbing ability or revive the sulfur-poisoned catalyst. For these reasons, the air-fuel ratio control system is required to expand the air-fuel ratio measurable range and enhance the accuracy of measuring the air-fuel ratio within such a range.
In general, oxygen sensors are known as air-fuel (A/F) sensors to determine an exhaust gas air-fuel ratio. Such sensors are of two types: one is a cup-shaped A/F sensor and the other is a laminated A/F sensor (also called a multilayered A/F sensor). The cup-shaped A/F sensor is equipped with a sensor element made up of a cup-shaped solid electrolyte body, a pair of electrodes affixed to outer and inner surfaces of the solid electrolyte body, and a diffusion layer surrounding the solid electrolyte body. The solid electrolyte body also has installed in an inner chamber thereof a bar heater which serves to heat the whole of the sensor element to kept it in a desired activate condition. The inner chamber of the solid electrolyte body forms an air duct leasing to the atmosphere.
The laminated A/F sensor is equipped with a sensor element made of a strip-shaped lamination of a solid electrolyte body, a diffusion layer, and an insulating layer which defines an air duct. The solid electrolyte body has affixed thereto a pair of electrodes which are opposed to each other. The insulating layer has a heater embedded therein.
Comparing between the cup-shaped A/F sensor and the laminated A/F sensor structurally, we found the following problems and advantages. The volume or mass of a portion of the sensor element of the cup-shaped A/F sensor heated for activation is greater than that of the laminated A/F sensor, thus resulting in an increased time required to complete the activation at cold engine start-up, a difficulty in activating the sensor element early, and an increased electric power consumed by the heater. In contrast, the laminated A/F sensor has the advantages that it is easy to install the heater integrally in the sensor element and to decrease the volume of the sensor element to accelerate the activation thereof, which results in a decrease in electric power consumed by the heater. For these reasons, the laminated A/F sensors have become prevalent.
If the size of the sensor element of the laminated A/F sensors is decreased, the volume of the air duct needs to be decreased. The decrease in volume of the air duct requires decreasing an electric current flowing through the sensor element (will also be referred to as a sensor element current below). Specifically, when exhaust gasses of the engine are rich, the sensor is so controlled that oxygen (O2) contained in air within the air duct is pumped into a gas chamber of the sensor element filled with the exhaust gasses. If the sensor element current is great, it causes a pumped amount of the oxygen to increased, which requires the need for increasing the size of the air duct. It is, thus, necessary for decreasing the size of the sensor element to decrease the sensor element current. This may be achieved by decreasing the size of the electrodes or increasing a diffusion rate of the diffusion layer (e.g., decreasing the porosity of the diffusion layer).
FIG. 16 illustrates an example of conventional electric circuits designed to measure the sensor element current flowing through the laminated A/F sensor.
A reference voltage source 153 is connected to a positive (+) terminal of a sensor element 150 through an operational amplifier 151 and a current-measuring resistor 152. A voltage applying control circuit 155 is connected to a negative (−) terminal of the sensor element 150 through an operational amplifier 154. The voltage appearing at a terminal A leading to an end of the resistor 152 is kept identical with the reference voltage Vƒ. The sensor element current flows through the current-measuring resistor 152 to change the voltage appearing at a terminal B. For instance, when the exhaust gasses are on the lean side, the current flows from the positive terminal to the negative terminal of the sensor element 150, so that the voltage appearing at the terminal B rises. Alternatively, when the exhaust gasses are on the rich side, the current flows from the negative terminal to the positive terminal of the sensor element 150, so that the voltage appearing at the terminal B drops. The voltage application control circuit 155 works to monitor the voltage at the terminal B and determine the voltage to be applied to the sensor element 150 (i.e., the voltage at a terminal D) as a function of the monitored voltage. The voltage at the terminal B is outputted as indicating the air-fuel ratio to a microcomputer (not shown) through an operational amplifier 156.
Keeping the sensor element 150 in a desired activate condition requires bringing an ac impedance Zac of the sensor element 150 into agreement with a given target value. Energization of a heater installed in the sensor element 150 is, thus, controlled as a function of a deviation of the impedance Zac from the target value. The determination of the impedance Zac is achieved by sweeping the voltage developed at the terminal D in an ac form through the voltage application control circuit 155, measuring a change in voltage ΔV at the terminal D, calculating a current change ΔI derived by dividing a change in voltage at the terminal B by a resistance value of the resistor 152, and dividing the voltage change ΔV by the current change ΔI(i.e., Zac=ΔV/ΔI).
Differences in sensor characteristic and solution in determining the air-fuel ratio between the above described cup-shaped A/F sensor and the laminated A/F sensor will be discussed below. It is assumed that an air-fuel ratio measurable range of the A/F sensors is between an air-fuel ratio of 11 to an air-fuel ratio in the atmospheric air (which will also be referred to as a free-air ratio below).
It is assumed that the cup-shaped A/F sensor is designed to meet electrical specifications as shown below.
When the exhaust gasses show the free-air ratio, the sensor element produces a current of 2.5 mA. When the A/F ratio is 11, the sensor element produces a current of −13 mA. The impedance Zac is 22Ω. The dc internal resistance Ri is 30Ω. The sensor control circuit, as illustrated in FIG. 16, has the following electrical specifications. A change in voltage used to measure the impedance Zac is ±0.3V. A current-measuring resistance is 63Ω. The reference voltage Vƒ is 2.5V.
When the exhaust gasses have a stoichiometric air-fuel ratio, the voltage appearing at the terminal B will be identical with that at the terminal A. An sensor output of the operational amplifier 156, i.e., the voltage at the terminal B has values, as indicated below, at the free-air ratio and an air-fuel ratio of 11, respectively.Output (free-air ratio)=2.5V+63Ω×25 mA=4.075VOutput(A/F=11)=2.5V+63Ω×(−13 mA)=1.681V
If the sensor output is inputted to a microcomputer through a 10-bit A/D converter to determine the A/F ratio, the measurement resolution within a range of an air-fuel ratio of 11 to the free-air ratio is(4.075−1.681)/5V×1024=490
If a change in the sensor element current per air-fuel ratio of one near the stoichiometric air-fuel ratio is 2 mA, the measurement resolution is2 mA×63Ω/5V×1024=490
The determination of the impedance Zac is achieved by sweeping the voltage at the terminal D to negative and positive sides. The voltage appearing at the terminal B undergoes changes, as shown below, at the free-air ratio and an air-fuel ratio of 11 due to the change in voltage at the terminal D to the positive side.Voltage at B=4.075V+63Ω×(0.3V/22Ω)=4.934VVoltage at B=1/681V+63Ω×(0.3V/22Ω)=2.54V
The change in voltage at the terminal D to the negative side results in a change in voltage at the terminal B. When the air-fuel ratio is 11, the voltage at the terminal B has a minimum value as shown below.Voltage at B=1.681V+63Ω×(−0.3V/22Ω)=0.822V
It is found that when the voltage at the terminal D is changed to the positive and negative sides to determine the impedance Zac, a resultant value of the voltage at the terminal B lies within an operational voltage range (0 to 5V) of the A/D converter of the microcomputer. Specifically, the sensor control circuit, as shown in FIG. 16, is so designed that the impedance Zac can be determined correctly.
The laminated A/F sensor is, as described above, so designed to decrease the sensor element current. For instance, the sensor element current produced in the laminated A/F sensor is approximately one-tenth of that in the cup-shaped A/F sensor.
It is assumed that the laminated A/F sensor is so designed to meet electrical specifications as shown below.
When the exhaust gasses show the free-air ratio, the sensor element produces a current of 2.5 mA. When the A/F ratio is 11, the sensor element produces a current of −1.3 mA. The impedance Zac is 28Ω. The dc internal resistance Ri is 60Ω. The sensor control circuit, as illustrated in FIG. 16, has the following electrical specifications. A change in voltage used to measure the impedance Zac is ±0.3V. A current-measuring resistance is 185Ω. The reference voltage Vƒ is 2.5V.
When the exhaust gasses have a stoichiometric air-fuel ratio, the voltage appearing at the terminal B will be identical with 2.5V at the terminal A. An sensor output of the operational amplifier 156, i.e., the voltage at the terminal B has values, as indicated below, at the free-air ratio and an air-fuel ratio of 11, respectively.Output (free-air ratio)=2.5V+185Ω×2.5 mA=2.9625VOutput (A/F=11)=2.5V+185Ω×(−1.3 mA)=2.2595V
It will be apparent from the above that the laminated A/F sensor is so designed that the voltages at terminals B and D can be measured correctly to determine the impedance Zac. If the voltage at the terminal D is changed by 0.3V to the positive side to determine the impedance Zac, the voltage developed at the terminal B has values, as indicated below, at the free-air ratio and an air-fuel ratio of 11, respectively.Voltage at B=4.9446VVoltage at B=4.2416V
If the voltage at the terminal D is changed by 0.3V to the positive side to determine the impedance Zac, the voltage developed at the terminal B has a minimum value, as shown below, at an air-fuel ratio of 11.Voltage at B=0.277V
If the sensor output is inputted to a microcomputer through a 10-bit A/D converter to determine the A/F ratio, the measurement resolution within a range of an air-fuel ratio of 11 to the free-air ratio is(2.9625−2.2595)/5V×1024=144
It is found that the measurement resolution of the laminated A/F sensor is approximately 0.3 times that of the cup-shaped A/F sensor (i.e., 144/490=0.294).
If a change in the sensor element current per air-fuel ratio of one near the stoichiometric air-fuel ratio is 0.2 mA, the measurement resolution is0.2 mA×185Ω/5V×1024=7
It is found that the measurement resolution of the laminated A/F sensor is approximately 0.3 times that of the cup-shaped A/F sensor (i.e., 7/25=0.28).
The reasons why the measurement resolution of the laminated A/F sensor is lower than that of the cup-shaped A/F sensor will be discussed below.
The sensor element current produced in the laminated A/F sensor is, as described above, decreased to approximately one-tenth of that of the cup-shaped A/F sensor. Thus, if the air-fuel ratio measurable range is between an air-fuel ratio of 11 and the free-air ratio, a range of the sensor element current in the cup-shaped A/F sensor is between −13 mA and 25 mA (=38 mA). A range of the sensor element current in the laminated A/F sensor is between −1.3 mA and 2.5 mA (=3.8 mA). An ac current produced when the impedance Zac is measured in the cup-shaped A/F sensor is 13.6 mA (=0.3V/22Ω). An ac current produced when the impedance Zac is measured in the laminated A/F sensor is 10.7 mA (=0.3V/28Ω). The current produced to measure the impedance Zac in the cup-shaped A/F sensor is 35.8% (13.6 mA/38 mA=0.358) of the sensor element current produced to measure the air-fuel ratio. The current produced to measure the impedance Zac in the laminated A/F sensor is 281.6% (10.7 mA/3.8 mA=2.816) of the sensor element current produced to measure the air-fuel ratio.
It is found that a ratio of the current to measure the impedance Zac to the sensor element current to measure the air-fuel ratio in the laminated A/F sensor is much greater than that in the cup-shaped A/F sensor. The resistance value of a resistor (i.e., the resistor 152 in FIG. 16) used to measure the sensor element current in the laminated A/F sensor must, therefore, be set smaller than that in the cup-shaped A/F sensor, which results in decreased resolution in determining the air-fuel ratio.
If the dc internal resistance value (or ac impedance Zac) of the sensor element of the laminated A/F sensor is increased, it will result in a decrease in current to measure the impedance Zac, so that a ratio of the current to measure the impedance Zac to the sensor element current to measure the air-fuel ratio is decreased. The increase in dc internal resistance of the sensor element, however, results in a change in sensor characteristic (see FIG. 3), which leads to a difficulty in controlling the voltage applied to the sensor element to measure the air-fuel ratio accurately. It is, thus, advisable that the dc internal resistance of the sensor element be not changed.
Japanese Patent First Publication No. 11-37971 teaches techniques for improving resolution in measuring the air-fuel ratio within a wide range. FIG. 17 illustrates a sensor control circuit installed in a gas concentration measuring apparatus as disclosed in the publication. The same reference numbers as employed in FIG. 16 refer to the same parts, and explanation thereof will be omitted here.
Two resistors 161 and 162 are connected in series to measure the current produced in the sensor element 150 (i.e., the sensor element current). A switch 163 is installed which establishes one of connections of the operational amplifier 156 to the terminals B and C selectively as a function of an instantaneous value of the air-fuel ratio. Specifically, when exhaust gasses have at the free-air ratio, the switch 163 closes between the operational amplifier 156 and the terminal C. The sensor element current is measured through the resistor 161 and outputted through the operational amplifier 156. When the exhaust gasses have the stoichiometric air-fuel ratio, the switch 163 closes between the operational amplifier 156 and the terminal B. The sensor element current is measured through the resistors 161 and 162 and outputted through the operational amplifier 156. This ensures the accuracy of measuring the air-fuel ratio over the wide range and improves it especially within a range in the vicinity of the stoichiometric air-fuel ratio.
The structure of FIG. 17, however, has the drawback in that in a case where the laminated A/F sensor is employed, a ratio of the current to measure the impedance Zac to the sensor element current to measure the air-fuel ratio is much greater than that in the cup-shaped A/F sensor, thus resulting in decreased resolution in determining the air-fuel ratio. The problem of the measurement resolution decreasing with a decrease in sensor element current is still standing unsolved.